1. Field of the Invention
The invention relates to a piston for use in an internal combustion engine. More specifically, the invention relates to a piston whose skirt has a sliding surface on which a layer of a coating material is formed.
2. Description of the Related Art
A piston for use in an internal combustion engine is usually provided with a piston skirt (hereinafter referred to simply as “a skirt”) to suppress the oscillation of the piston within the cylinder bore. The skirt slides along an inner wall surface of the cylinder bore (hereinafter referred to as a cylinder wall) and is flexibly deformed, thereby restraining the friction between the piston and the cylinder wall from increasing while suppressing the oscillation of the piston.
To reduce the friction between the piston and the cylinder wall, the skirt is required to be flexibly deformed to an appropriate degree, and is usually constructed of a thin wall body. To reinforce an end portion of the sliding surface of the skirt in a circumferential direction of the piston (hereinafter referred to as a sliding surface circumferential end portion), the piston is generally provided with a connection wall that connects the sliding surface circumferential end portion to a piston pin boss. These components, namely, the connection wall, the skirt, and the piston pin boss extend from the piston head. In the following description, the region of the end of the skirt that is most distant from the piston head is referred to as “a tail” of the skirt. The central portion of the sliding surface of the skirt in the circumferential direction of the piston is referred to as “a sliding surface central portion”.
In the piston thus constructed, with a view to reducing the sliding resistance between the skirt and the cylinder wall, it is known to form a layer of a coating material such as resin or the like on the sliding surface of the skirt. Furthermore, an uncoated groove-like section may be formed in the sliding surface, into which lubricating oil is introduced (e.g., see Japanese Utility Model Application Publication No. 4-88750 (JP-U-4-88750) and Japanese Patent Application Publication No. 2005-320934 (JP-A-2005-320934)).
In the piston described in Japanese Utility Model Application Publication No. 4-88750 (JP-U-4-88750), a layer of a resin coating material is formed on a sliding surface that slides along the cylinder wall. Furthermore, in the piston of Japanese Utility Model Application Publication No. 4-88750 (JP-U-4-88750), a section where the layer of the resin coating material is not formed (a non-coated portion) extends like a groove from a tail side of a sliding surface circumferential end portion toward the top face of the piston head of a central portion of a skirt in a circumferential direction thereof. By providing this non-coated groove-like portion, oil adhering to the cylinder wall is introduced into the non-coated portion as the piston reciprocates, and is supplied to the sliding surface.
In a piston described in Japanese Patent Application Publication No. 2005-320934 (JP-A-2005-320934), convex sections where a layer (coat) of a solid lubricating material containing molybdenum disulfide and the like exists and concave sections where the layer (coat) of the solid lubricating material does not exist are alternately provided along a sliding surface (outer peripheral surface) of a skirt. Lubricating oil is retained in the concave sections and supplied to the convex sections to thereby enhance the abrasion resistance of the sliding surface of the skirt and reduce the sliding resistance thereof.
A piston for an internal combustion engine is deformed through the heat received during operation of the internal combustion engine. In consideration of this thermal deformation, a skirt of the piston is formed, in a cold condition, in the shape of an ellipse having a minor axis in a direction in which a pin hole of a piston pin boss extends (hereinafter referred to as a piston pin axial direction) and a major axis in a direction in which a thrust force and a counter thrust force act (hereinafter referred to as a thrust direction). The piston thus assuming the shape of the ellipse in the cold condition receives heat from a gap between corresponding ones of a plurality of cylinder bores formed in the internal combustion engine, expands in the piston pin axial direction. As a result, the shape of the piston becomes substantially circular.
A skirt of this piston is constructed such that a top portion of a sliding surface of the skirt is located closer to a central axis of the piston than a tail of the skirt. The top portion of an outer peripheral surface of the skirt becomes deformed outward in a radial direction of the piston due to the heat received from the top portion during operation of the internal combustion engine. The contact pressure that the outer peripheral surface of the skirt receives from a cylinder wall can thereby be made homogeneous.
If an internal combustion engine having a piston constructed as described above is operated at a low rotational speed or under a low load, especially on a top portion side of a circumferential end portion of a sliding surface (hereinafter referred to as a sliding surface circumferential end portion), the gap (hereinafter referred to as a clearance) between the sliding surface circumferential end portion and a cylinder wall is larger with a larger oil film thickness in comparison with other regions of an outer peripheral surface of a skirt. When there is a region having a thicker oil film than the other regions as described above, there is caused a problem in that the shearing resistance of oil is strong, namely, that the sliding resistance on the cylinder wall is strong. Further, owing to the large clearance between the outer peripheral surface of the skirt and the cylinder wall, there is also caused a problem in that a loud slapping sound is produced upon abutment of the outer peripheral surface of the skirt on the cylinder wall.
When the internal combustion engine is operated at a high rotational speed or under a high load, the clearance between a sliding surface central portion and the cylinder wall is smaller and the oil film is thinner in comparison with the other regions of the outer peripheral surface of the skirt, due to thermal expansion of the piston in the thrust direction. Thus, there is an increased likelihood that the sliding surface central portion may suffer abrasion or seizure.
Accordingly, in the piston for use in the internal combustion engine, there is a demand for an art capable of making the thickness of the oil film on the outer peripheral surface of the skirt as homogeneous as possible by introducing lubricating oil on the sliding surface circumferential end portion of the outer peripheral surface of the skirt to the sliding surface central portion.